'Scuse me for giving JRPG's credit here... well, the older ones anyway. Best ending, Final Fantasy VIII, the cinematic at the end was epic. Another good one was FFVII:Crisis Core, the ending was the first time in video game history that they made the Ragnaught cry... seeing Zack dying and him telling cloud that "You're my living legacy"... epic.
Worst Ending, Final Fantasy VII, considering how well the story went, the ending seemed somewhat rushed, an extra month of work would have gone a long way with sorting out the ending, still the best RPG of all time hands down though. Halo 2 and Halo 3 had crap endings and I've got to say I did not like the portal ending, loved the game but the ending made it feel just too short... I don't know, might be how I think but yeah. GTA 3 and Vice City had crap endings as did the original Lion King game on DOS... that's right I had a copy of that, what are you going to do, laugh?
Ragnaught